Use of limited proteolysis and MALDI-MS to determine the "minimal domains" of the Drosophila TAFS p42 and p62. The information was used to design constructs of the protein that yielded crystals which diffracted to high resolution by X-ray analysis. Heavy-atom constructs (selenomethionine and Hg) were used in the X-ray analysis. Conventional biochemical analyses of the heavy-atom constructs were not feasible. The heavy-atom constructs were characterized by MALDI-MS and ESI-MS. Electrospray-MS directly measured the recombinant truncated p42/p62 complex sprayed from solution into in the gas phase, representing the first direct measurement of the heterodimer.